(a) Field of the Invention
The present invention relates to a carrier to interference-plus-noise ratio (CINR) estimating method and device of an orthogonal frequency division multiplex (OFDM) scheme. More particularly, the present invention relates to a device and method thereof that are capable of efficiently estimating the CINR when a preamble subcarrier or a pilot subcarrier transmit power is different from a data subcarrier transmit power in the OFDM system, the respective base stations uses preamble subcarriers whose positions are different, or a data area is partially loading.
(b) Description of the Related Art
In a fourth generation mobile communication system requiring high capacity data transmission such as a wireless local area network (WLAN), wireless broadcasting, or Digital Multimedia Broadcasting (DMB), the OFDM system has been used so as to transmit wideband high speed data. The OFDM system divides a bandwidth into a plurality of subcarrier and transmits the divided bandwidth, and the basic function of OFDM is to convert serially input data streams into N parallel data items, respectively load the N parallel data items on the independent subcarriers, and transmit them so as to increase a data rate.
FIG. 1 is a block diagram of a data transmitting/receiving system using an OFDM system.
An OFDM transmit system includes an encoder 102, a serial/parallel (S/P) converter 104, a preamble or pilot generator 106, a multiplexer 108, an inverse fast Fourier transform (IFFT) converter 110, a parallel/serial (P/S) converter 112, and a digital/analogue (D/A) converter and filter 114, and a receive system includes an analogue/digital (A/D) converter and filter 116, an S/P converter 118, a fast Fourier transform (FFT) converter 120, a demultiplexer 122, a preamble or pilot extractor 124, a P/S converter 126, and a decoder 128.
The encoder 102 modulates data to be transmitted into a desired modulation scheme, for example, binary phase shift key (BPSK), quadrature phase shift key (QPSK), 16 QAM (Quadrature Amplitude Modulation), and 64 QAM schemes.
The S/P converter 104 converts serially received high-speed data into low-speed parallel data, and the preamble or pilot generator 106 generates a pilot or preamble to be loaded on the transmit data. The multiplexer 108 multiplexes the transmit data received from the S/P converter 104 with the preamble or pilot generated by the preamble or pilot generator 106.
The IFFT converter 110 converts the multiplexed data into time-axis signals, and the P/S converter 112 converts the parallel signal into serial data and adds a cyclic prefix to a front of the converted data.
The D/A converter and filter 114 converts a digital format of the transmit data that is converted into the serial signal by the P/S 112 into an analog signal, filters the converted data, and transmits the same to an antenna of an RF unit.
An antenna of the receiving system receives the transmitted analog signal, and the A/D converter and filter 116 filters and converts the analog signal into the digital signal. The S/P converter 118 eliminates the cyclic prefix and converts the digital signal into the parallel signal and transmits the converted signal to the FFT converter 120, the FFT converter 120 performs a Fourier transform on the transmitted parallel signal and transmits the transformed signal to the demultiplexer 122, the demultiplexer 122 demultiplexes the data, and then the preamble or pilot extractor 124 separates a preamble or pilot signal from the data. The P/S converter 126 converts the separated parallel data signal into the serial signal. The decoder 128 demodulates data using a channel estimating value estimated by the preamble or pilot signal extracted from the preamble or pilot extractor 124.
FIG. 2 is a block diagram of a data transmitting/receiving system using an OFDM system.
The data multiplexed in the multiplexer 108 and transmitted to the receiving system includes a data symbol directly including the preamble and the data.
The preamble includes frame synchronization, cell search, time/frequency synchronization, and channel estimating information. Generally, the preamble is placed at the front of the frame, but it may be placed at the middle or rear thereof.
A mobile communication system such as a high-speed downlink packet access (HSDPA) and an evolution data only (1x EV-DO) system adopts an adaptive modulation and coding scheme (AMC) that is capable of changing the modulation scheme and a channel coding rate according to a channel environment, so as to increase a data rate, and has various modulation and coding scheme (MCS) levels that are selectable according to the channel environment. At this time, in order to select an accurate MCS level, instantaneous channel CINR is estimated.
The CINR may be directly estimated using data, and it may be indirectly estimated using a preamble by applying the preamble to the data area.
FIG. 3 is a block diagram for showing a conventional CINR estimating algorithm.
According to the conventional CINR estimating method using a preamble, differences between adjacent subcarriers using an operator are obtained from the receive signal rm (S302), the differences are averaged to estimate an interference power (S304), and the inverse estimated interference power is obtained (S306).
In addition, a predetermined number of moving averages are calculated from the received signal (S308), and the moving averages are averaged to estimate a signal power (S310). In addition, the CINR is calculated from the signal power and the interference power estimated through S306 to S310 (S312).
According to the CINR calculating method using data, the high CINR area has good accuracy. However, the low CINR area in which the received data reliability is deteriorated has considerably reduced accuracy.
In addition, according to the CINR calculating method using a preamble, good CINR estimating accuracy is provided. However, the respective base stations must have a preamble sequence placed on the subcarriers having the same position, and also errors may occurs at the high CINR area when the preamble transmit power is different from the data transmit power.
The preamble has all adjacent cells always transmitted, and accordingly it is used to estimate the CINR to thereby determine the data area MCS. However, in the case of partial loading, that is, the adjacent cells partially use a subcarrier, the preamble CINR is always less than that of the real data area. Accordingly, an optimum MCS level cannot be determined.
FIGS. 4 and 5 illustrate preamble sequence allocation in which a subcarrier transmits a preamble in a cellular system.
FIG. 4 illustrates a cellular system having a cell 0 including a receiver for receiving data and six adjacent cells, in which receive signals are interfered with by the adjacent cells.
OFDM uses various preamble and data area subcarrier allocation methods. Generally, the preamble is transmitted every N number of subcarriers, is selected at a sequence having a low peak to average power ratio (PAPR), and is transmitted while amplifying the power in comparison with the data symbol.
In FIG. 5, the preamble sequence is transmitted every 4 subcarriers. The subcarrier transmission interval or a start point of the transmitted subcarrier may be variously established. At this time, Pmq(n) is given as the preamble sequence used at the respective base stations, q is given as a cell number, m is given as a subcarrier number, and n is given as a symbol number.
As shown in FIG. 5, when the preamble is used at the same subcarriers for all the cells, and the preamble transmission power is the same as the data transmission power, the CINR can be accurately estimated. However, when the preamble transmission power is amplified to be greater than the data transmission power, in the case of the high CINR area, an error occurs between the CINR estimated at the preamble and the CINR of the data symbol.
In order to enhance cell search performance or channel estimating performance using a preamble, the respective cells can use the subcarriers differently within the preamble symbol. Such a method has been applied to the IEEE 802.16 (a wideband wireless communication standard). There is a problem in that the CINR value estimated using a subcarrier is different from the data area CINR value determined when all the subcarriers are used, when the subcarriers of the preambles used in the respective cells are different.
FIGS. 6 and 7 respectively illustrate a cellular system in which a preamble sequence is allocated when respective cells transmit a preamble using a different subcarrier.
FIG. 6 illustrates segments having a segment a, a segment b, and a segment c used at preambles of the respective cells using IEEE 802.16.
In addition, FIG. 7 illustrates subcarriers using preambles at a cell structure used in FIG. 6. The interference to be estimated at the preamble structure can only estimate Cell 2, Cell 4, and Cell 6 when such subcarriers are used. Therefore, the interference of Cell 1, Cell 3, and Cell 5 may not be estimated, and the preamble CINR may be estimated to be higher than the CINR of the real data area. Accordingly, there is a problem in that such an error is variously changed according to the segments used at the preamble.
In addition, there is a problem in that an error occurs between the CINR using the preamble and the data area CINR when the data area is partially loaded since the CINR estimating method using the preamble does not regard the data loading, and it is performed if the subcarriers are always allocated for all the data areas.
In order to measure a signal-to-noise ratio (SNR) in such an OFDM system, Korea Patent Application No. 10-2003-0039580 entitled “A Signal-to-noise ratio (SNR) Measuring Method and Apparatus using a Repeated Signal in an OFDM”, an IEEE PIMRC paper entitled “Experimental Evaluation Throughput Performance in Broadband Packet Wireless Access Based on VSF-OFCDM and VSF-CDMA” (2003), and an IEEE GLOBECOM paper entitled “Novel Noise Variance and SNR Estimation Algorithm for Wireless MIMO OFDM” (2003) disclose a signal-to-noise ratio (SNR) measuring method using a repeated signal, two adjacent subcarriers of a pilot symbol, and two consecutive training symbols. However, the CINR has not been estimated when the subcarriers used by the preambles of the respective cells are different and the data are partially loading.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.